Method and Device for Tunable Optical Filtering

ABSTRACT

An optical device includes an optical splitter having an input port, a first output port, a second output port and a resonant structure including at least a resonator, the optical splitter being adapted to receive at the input port a WDM optical signal and to output at the first and second output ports, respectively, a first and a second portion of the optical signal, the second portion including the channels lying on a sub-grid of optical frequencies spaced by an integer multiple of the WDM frequency spacing; an optical combiner having a first input port, a second input port, an output port and adapted to receive at the first and second input ports, respectively, the first and the second portions and adapted to output them at said output port; a first optical path optically connecting the first output port of the optical splitter to the first input port of the optical combiner so as to propagate the first portion; a second optical path optically connecting the second output port of the optical splitter to the second input port of the optical combiner so as to propagate the second portion; and an optical filter optically coupled to the second optical path, wherein the optical combiner includes at least one resonant structure including at least a resonator.

FIELD OF THE INVENTION

The present invention relates to the field of optical communicationsystems including tunable optical filtering functionality, such astunable optical add and/or drop functionality.

BACKGROUND OF THE INVENTION

A common technique to increase the transmission capacity of todayoptical communication systems is wavelength division multiplexing (WDM),wherein a plurality of optical channels, each having a respectiveoptical frequency (and correspondingly respective optical wavelength),are multiplexed together in a single optical medium, such as for examplean optical fiber. The optical frequencies allocated for the WDM channelsare typically arranged in a grid having an equal spacing between twoadjacent frequencies. In dense WDM (DWDM), wherein the WDM channels maybe closely spaced, the frequency spacing is typically equal to about 100GHz (corresponding wavelength spacing of about 0.8 nm) or about 50 GHz(about 0.4 nm). Other used channel separations are 200 GHz, 33.3 GHz and25 GHz. Typically, the set of allocated optical frequencies occupies anoptical bandwidth of about 4 THz, which gives room for the use of up to40 or 41 WDM channels having 100 GHz spacing. The device of the presentinvention is suitable for a WDM optical bandwidth of at least about 2THz, preferably at least about 3 THz, typically placed around 1550 nm.

Optical networking is expected to be widely used in perspective opticalcommunication field. The term ‘optical network’ is commonly referred toan optical system including a plurality of point-to-point orpoint-to-multipoint (e.g., metro-ring) optical systems opticallyinterconnected through nodes. In all-optical transparent networks few orno conversions of the optical signal into electrical signal, and thenagain in optical signal, occur along the whole path from a departurelocation to a destination location. This is accomplished by placing atthe nodes of the optical networks electro-optical or optical deviceswhich are apt to process the optical signal in the optical domain, withlimited or no need for electrical conversion. Examples of such devicesare optical add and/or drop multiplexers (OADM), branching units,optical routers, optical switches, optical regenerators (re-shapersand/or re-timers) and the like. Accordingly, the term ‘opticalfiltering’ or ‘optical processing’, for the purpose of the presentdescription is used to indicate any optical transformation given to anoptical radiation, such as extracting a channel or a power portion ofsaid channel from a set of WDM channels (‘dropping’), inserting achannel or a power portion of said channel into a WDM signal (‘adding’),routing or switching a channel or its power portion on a dynamicallyselectable optical route, optical signal reshaping, retiming or acombination thereof. In addition, optical systems, and at a greaterextent optical networks, make use of optical amplifiers in order tocompensate the power losses due to fiber attenuation or to insertionlosses of the optical devices along the path, avoiding the use of anyconversion of the optical signal into the electrical domain even forlong traveling distances and/or many optical devices along the path. Incase of DWDM wavelengths, all channels are typically optically amplifiedtogether, e.g. within a bandwidth of about 32 nm around 1550 nm.

In optical systems, and at a greater extent in optical networks, aproblem exists of filtering one or more optical channels at the nodeswhile minimizing the loss and/or the distortion of the filtered opticalchannel(s), as well the loss and/or the distortion of the opticalchannels transmitted through the node ideally without being processed(hereinafter referred to as ‘thru’ channels). Advantageously, theoptical processing node should be able to simultaneously process morethan one channel, each one arbitrarily selectable independently from theother processed channels. Ideally up to all the channels may besimultaneously selectable to be processed, but in practice a numberbetween 2 and 16, preferably between 4 and 8, is considered to besufficient for the purpose.

It is desirable that the optical processing node is tunable orreconfigurable, i.e., it can change dynamically the subset of channelson which it operates. In order to be suitable to arbitrarily select thechannel to be processed within the whole WDM optical bandwidth, thetuning range of the whole optical processing node should be at leastequal to said optical bandwidth. It is in general a problem to tune anoptical filter over the whole optical bandwidth, especially when thebandwidth exceeds about 3 THz, for example when it is equal to about 4THz. For example, notwithstanding the silicon's fairly largethermo-optic effect, scanning the entire telecommunication C-band (32 nmor 4 THz) with a single tunable silicon filter, such as a single siliconmicroring filter, remains quite a difficult task due to the hightemperatures reached at the heater layer (up to about 600° C.).

It is also preferred that while the processing node “moves” from aninitial channel (A) to a destination channel (B), the channels differentfrom A and B remain unaffected by the tuning operation. In this case thecomponent is defined as ‘hitless’. In particular, the channels placedbetween the initially processed channel and the final channel aftertuning should not be subject to an additional impairment penalty, called‘hit’, by the tuning operation. The hit may include a loss penaltyand/or an optical distortion such as phase distortion and/or chromaticdispersion.

For example, optical communication networks need provisions forpartially altering the traffic at each node by adding and/or droppingone or several independent channels out of the total number. Typically,an OADM node removes from a WDM signal a subset of the transmittedchannels (each corresponding to one frequency/wavelength), and adds thesame subset with a new information content, said subset beingdynamically selectable.

There are several additional concerns. The tunable optical processingnode should not act as a narrow band filter for the unprocessedchannels, since concatenation of such nodes would excessively narrow thechannel pass bands. The tunable optical processing node should also beultra-compact and should have low transmission loss and low cost, sincethese important factors ultimately determine which technology isselected.

U.S. Pat. No. 6,839,482 discloses (see, e.g., FIG. 2 thereof) an opticalfilter device for processing a multi-frequency light signal to separatetherefrom a predetermined frequency component, the device comprising:(i) a first tunable filter structure having a first tuning range andoperable to receive an input light signal and output first and secondlight components thereof through first and second spatially separatedlight paths, respectively, the first light component having a specificfrequency range of the input signal including said predeterminedfrequency component, and the second light component including aremaining portion of the input light; and (ii) a second tunable filterstructure having a second tuning range defining an optical spectrumoverlapping with that of the first filter, the second filter beingoperable to receive the first light component and separate therefromsaid predetermined frequency component to propagate to a drop/add lightpath of the device and direct a remaining portion of the first lightcomponent into the first filter structure to be output at the secondlight path.

SUMMARY OF THE INVENTION

The Applicant has discovered that in U.S. Pat. No. 6,839,482 the channelresonant with the first filter structure are distorted twice in theinteraction with the first filter structure and the remainingnon-resonant channels are negligibly distorted. The thru output isconsequently strongly not equalized and the dispersion response of thefilter is penalized.

The Applicant has also noted that the filter device described in thecited patent is not optimally designed for adding and/or dropping aplurality of independent optical channels. Considering, by way ofexample, the need of adding and/or dropping two independent channelsfrom a WDM signal, in the cited patent it is suggested to cascade twotimes the whole structure (e.g. that of FIG. 2, bottom, thereof), thusgiving rise to several disadvantages. The resulting structure would becomplex, both in structure and in operation. Moreover, the cascade oftwo first tunable filter structures (e.g. ring-resonator pairs R1-R2 andR3-R4 of FIG. 2 of the cited patent) gives rise to a correspondingduplication of the attenuation and the chromatic dispersion introducedby the single first tunable filter structure on the thru channels. Theabove problems worsen with the increasing of the number of independentchannels to be added and/or dropped.

The Applicant has found that there is a need for an opticalcommunication system having tunable optical processing functionalitywhich leaves unaltered, or at least reduces the alteration of, the thruchannels during processing operation. Moreover, the optical processingnode should preferably leave unaltered the thru channels during tuning,i.e. hitless. In particular, it is desired that the optical processingnode introduces no or low chromatic dispersion to the thru channels. Inaddition, the optical processing node should preferably be low-loss,low-cost, fast tunable and/or broadband.

The Applicant has found a method and a system for optical transmissionfurnished of optical processing functionality which can solve one ormore of the problems stated above. The solution of the present inventionis simple, feasible and low cost. A particular architecture has beenconceived which enables a full C-band (32 nm) tunability by effectivelytuning the single drop filter only about half of it (18.4 nm). Anadvantage of the particular architecture is that both the opticalsplitter and the optical combiner do not need to be tuned while havingthe capability of filtering an arbitrary optical channel. A trimmingheater may be fabricated on top of each device component to carefullyalign their frequencies to the ITU grid and to compensate for possiblefabrication errors.

According to an aspect of the present invention, an optical device asset forth in appended claim 1 is provided. Advantageous embodiments ofthe optical device as set forth in appended claims 2 to 13 are provided.

In a further aspect of the present invention, an optical communicationsystem as set forth in appended claim 14 is provided. The opticalcommunication system comprises a transmitter, a receiver, an opticalline optically connecting the transmitter and the receiver and anoptical device according to the above coupled to the optical line.

According to a still further aspect of the present invention, a methodfor filtering a WDM optical signal as set forth in appended claim 15 isprovided. Advantageous embodiments of this method as set forth inappended claims 16 to 29 are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be made clearby the following detailed description of an embodiment thereof, providedmerely by way of non-limitative example, description that will beconducted making reference to the annexed drawings, wherein:

FIG. 1 schematically shows in terms of functional blocks an exemplaryoptical communication system architecture according to the presentinvention;

FIG. 2 is a schematic diagram showing in terms of functional blocks ascheme for optical filtering according to the present invention;

FIG. 3 shows in terms of functional blocks an exemplary configuration ofa device for tunable optical add and/or drop multiplexing according tothe present invention;

FIGS. 4 and 5 respectively show the calculated amplitude and dispersionresponse of the optical splitter comprised in the filtering device ofFIG. 3;

FIGS. 6A and 6B respectively show the calculated amplitude anddispersion response of the optical add and/or drop filter comprised inthe device shown in FIG. 3;

FIGS. 7 and 8 respectively show the calculated amplitude and dispersionresponse of the optical add and/or drop multiplexing device shown inFIG. 3; and

FIGS. 9A and 9B are schematic diagrams exemplary showing the operationof the device shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE INVENTION

FIG. 1 shows an optical communication system architecture according to apossible embodiment of the present invention.

The optical communication system 100 comprises at least a transmitter110, a receiver 120 and an optical line 130 which optically connects thetransmitter and the receiver. The transmitter 110 is an opto-electronicdevice apt to emit an optical signal carrying information. It typicallycomprises at least an optical source (e.g., a laser) apt to emit anoptical radiation and at least a modulator apt to encode informationonto the optical radiation. Preferably, the transmitter 110 is a WDMtransmitter (e.g., a DWDM transmitter) and the optical signal maycomprise a plurality of optical channels (each carryingmodulation-encoded information) having respective optical frequenciesequally spaced by a given frequency spacing and occupying an opticalbandwidth. Preferably, said optical bandwidth is at least 2 THz (in thenear-infrared wavelength range, e.g. from 900 nm to 1700 nm), morepreferably it is at least 3 THz, still more preferably it is equal toabout 4 THz. The receiver 120 is a corresponding opto-electronic deviceapt to receive the optical signal emitted by the transmitter and todecode the carried information. The optical line 130 may be formed by aplurality of sections of optical transmission media, such as for exampleoptical fiber sections, preferably cabled. Between two adjacent sectionsof optical fiber, an optical or opto-electronic device is typicallyplaced, such as for example a fiber splice or a connector, a jumper, aplanar lightguide circuit, a variable optical attenuator or the like.

For adding flexibility to the system 100 and improving systemfunctionality, one or a plurality of optical, electronic oropto-electronic devices may be placed along the line 130. In FIG. 1 aplurality of optical amplifiers 140 are exemplarily shown, which may beline-amplifiers, optical boosters or pre-amplifiers.

According to the present invention, the optical system 100 comprises atleast one optical processing node (OPN) 150 optically coupled to theoptical line 130 and apt to filter or route or add or drop orregenerate, fully or partially, at least one optical channel of the WDMoptical signal propagating through the optical line 130. The OPNs arepreferably dynamically tunable or reconfigurable. In the particular casewherein the optical processing node 150 is an optical add/drop node 150,as shown in FIG. 1, i.e., a node adapted to route or switch or addand/or drop, the routed or switched or dropped or added channel(s) maybe received or transmitted by further receiver(s) 152 or transmitter(s)154, respectively, which may be co-located with the OPN node or at adistance thereof. The optical system or network 100 may advantageouslycomprise a plurality of optical processing nodes. In FIG. 1 a furtheroptical processing node 150′ is exemplarily shown, together with itsrespective optional transmitting and receiving devices 152′ and 154′.

An optical system 100 having optical add/drop nodes 150, as shown inFIG. 1, is commonly referred to as an optical network and it ischaracterized by having a plurality of possible optical paths for theoptical signals propagating through it. As exemplarily shown in FIG. 1,a number of six optical paths are in principle possible, whichcorresponds to all possible choices of the transmitter-receiver pairs inFIG. 1 (excluding the pairs belonging to the same node).

According to the present invention, the tunable optical processing node150 is suitable for independently filtering one or more optical channelswhile limiting the distortion of the thru channels, being based on ascheme in accordance with the following.

FIG. 2 shows a schematic diagram of an optical device 200 in accordancewith an embodiment of the present invention.

The general design scheme of the present invention comprises an opticalsplitter 210 with an input port 212 and a first 214 and a second 216output port and an optical combiner 220 having a first 222 and a second224 input port and an output port 226. Throughout the presentdescription, the terms ‘input’ and ‘output’ are used with reference to aconventional direction of propagation of the optical radiation (in FIG.2 exemplarily from left to right and from top to bottom, as indicated bythe thick arrows), but, when in operation, the optical radiation maypropagate in the opposite direction.

A first optical path 230 and a second optical path 240 optically connectin parallel configuration the optical splitter 210 to the opticalcombiner 220. The first optical path 230 connects the first output portof the optical splitter 210 to first input port of the optical combiner220. The second optical path 240 connects the second output port of theoptical splitter 210 to the second input port of the optical combiner220. The two optical paths 230 and 240 are preferably opticallyseparated unless in correspondence of the optical splitter and combiner210 and 220.

The optical splitter 210 is a resonant device, i.e. it comprises aresonant structure 218 which in turn comprises one or a plurality ofresonators (or resonant cavities) 218. (In the drawings, the symbolconsisting of three aligned points represent any arbitrary number ofelements of the type adjacent to the symbol). For example, theresonator(s) may be linear cavities (i.e. cavities each having aplurality of reflectors), microring resonators, racetrack resonators,photonic band gap cavities, Bragg gratings or the like. A singleresonant optical cavity has associated ‘resonant wavelengths’ (andcorresponding ‘resonant frequencies’), defined as those wavelengthswhich fit an integer number of times on the cavity length of theresonant optical cavity. For example a Bragg grating comprises aplurality of coupled resonant cavities. Strong frequency dependence ofthe phase/dispersion transfer function typically occurs incorrespondence of the resonant frequency(ies). The distance between twoadjacent resonant frequencies is referred to as the free spectral range(FSR) of the individual resonator.

The optical splitter 210 has an optical power response at the secondoutput port 216, when a broad spectrum optical radiation is inputted inits input port 212, periodically peaked with respect to the opticalfrequency, at least in an optical bandwidth of interest (e.g. 4 THzaround 1550 m or 193 THz). The distance between two successive peakswithin the optical power response function at the second output port 216is referred to as ‘free spectral range’ or FSR of the optical splitter210 and will be generally expressed in optical frequency units. Theoptical frequencies corresponding to the peaks of the optical powerresponse function at the second output port 216 are referred to as the‘resonances’ of the optical splitter 210 and they may, typically,corresponds to the resonances of one or more of the individualresonators.

The optical power response of the other output port 214 is typically thecomplementary function (1-f) of the optical power response above,neglecting the loss introduced by the optical splitter itself.

According to the present invention, the optical splitter 210 has a freespectral range corresponding to about an integer multiple of thefrequency spacing Δf of the allocated WDM frequencies (FSR=mΔf±40% Δf).The term integer multiple means an integer greater than or equal to two.Said integer multiple is preferably smaller than 10, more preferably itis comprised between, and including, 2 and 7. For example, given afrequency spacing of about 100 GHZ, the FSR is selected to be equal toabout 200 GHz or 300 GHz.

The ‘cross-talk’ of the optical splitter 210 is a known opticalparameter defined, at an output port, as the optical power level of anoptical channel adjacent to a given optical channel corresponding to apeak of the optical power response at that output port, expressed interm of relative optical power with respect to the power of the givenoptical channel. The cross-talk of the optical splitter 210 of thepresent invention is preferably low, e.g. it may be less than about −10dB, preferably less than about −15 dB, more preferably less than about−25 dB.

In other words, according to the present invention the optical splitter210 is apt to receive at its input port 212 a WDM optical signal havinga plurality of optical channels allocated on a WDM grid of n opticalfrequencies equally spaced by a given frequency spacing Δf and occupyingan optical bandwidth BW=(n−1)Δf, and to output at said first 214 andsecond 216 output port respectively a first and a second portion of saidoptical signal. The second portion substantially comprises all thechannels, within said plurality of channels, which are allocated on asub-grid (of the WDM grid) of optical frequencies equally spaced by aninteger multiple of said frequency spacing (mΔf) and the first portionsubstantially comprises the remaining channels. Here the term‘substantially’ is used to take into account the (typically inevitable)cross-talk described above. The optical splitter 210 is preferablyfurther selected so as to introduce low loss and/or low distortion (e.g.dispersion) to the split output channels. Assuming the n frequencies ofthe WDM grid being numbered with an index i from 1 to n, than the secondportion of optical channels substantially comprises all the channelshaving frequencies selected one every m frequencies of the WDM grid. Inthe special case of m=2, the second portion of optical channelssubstantially comprises all the channels having frequencies with a givenparity of the index i (e.g. the ‘odd’ channels), and the first portionof optical channels substantially comprises the remaining channelshaving the opposite parity (e.g. the ‘even’ channels).

In a configuration, as shown in FIG. 2, the optical splitter 210 maycomprise a first optical waveguide 211 optically connecting the inputport 212 to the first output port 214 and a second optical waveguide 213optically connected to the second output port 216. The resonantstructure 218 may be optically coupled to the first and second opticalwaveguide and optically interposed, with respect to the direction ofpropagation of the second portion, between the first and the secondoptical waveguide. In case of a resonant structure 218 comprising aplurality of resonators, they may be coupled in parallel between the twooptical waveguides 211 and 213 or, preferably, in series as shown inFIG. 2. Preferably the coupled resonators are less than four, morepreferably they are two or three. In case of two microring resonators218, as shown in FIG. 3, they typically have the same ring radius sothat the free spectral range of the individual resonators are all equaland they are also equal to the FSR, as defined above, of the wholeoptical splitter 210.

When the resonant splitter 210 is in operation, the optical channelsinput into the input port 212 which are output into the second outputport 216 coupled to the second optical path 240 (i.e. those channelsbelonging to the second portion) are those channels having opticalfrequencies which match the resonances of the resonating structure 218and they physically travel across the resonators 218, as indicated bythe down-arrow near microrings 218.

Preferably, a tuning device (not shown) is coupled to the opticalsplitter 210 in order to control the working point (i.e. the position ofthe resonances) of the optical splitter, such as for example in order toproperly match the peaks of the respective power response with the gridof the WDM frequencies and/or to compensate for possible fabricationerrors. Such working point may be controlled at a fixed position(‘trimming’) or it may be dynamically changed (‘tuning’), typicallywithin a tuning range equal to the FSR of the splitting device,depending on the operative conditions.

The optical combiner 220 is a combining device apt to receive in its twoinput ports 222 and 224 respectively two optical radiations (relatedrespectively to the first and second portion) propagating along thefirst and second optical path 230 and 240 and to combine them togetherso as to output the combined radiation into the output port 226,possibly with minimum loss and/or distortion.

The optical combiner 220 comprises one or a plurality of resonantstructures 225, 227, 229 each resonant structure comprising at least aresonator. According to the present invention, the one or more resonantstructures of the optical combiner 220 are apt, as a whole, to resonatewith the optical frequencies of the first portion so that, in operation,the first portion is output at said output port 226 by interaction withsaid at least one resonant structure 225, 227. On the other hand, the atleast one resonant structure 225, 227, 229 when configured to resonatewith the optical frequencies of the first portion, do not resonate withthe optical frequencies of the second portion, so that the latter doesnot interact (or negligibly interact) with the at least one resonantstructure of the combiner 220.

In one configuration, as shown in FIG. 2, the optical combiner 220comprises a plurality of resonant structures 225, 227, 229, wherein eachresonant structure comprises at least a resonator and it is apt toresonate with the optical frequencies of a respective sub-portion of thefirst portion so that, in operation, the respective sub-portion isoutput at said output port 226 by interaction with the respectiveresonant structure 225, 227, 229, being each respective sub-portiondifferent from the other sub-portions. Preferably, the FSR of theresonant structures are all substantially equal and further equal to theFSR of the splitter 210. Advantageously, the number of the plurality ofresonant structures 225, 227, 229 is equal to the integer multiple mdiminished by one unit, so that the corresponding (m−1) sub-portionsconstitute the first portion. Preferably, in operation, the grid ofresonances of each resonant structure 225, 227, 229, is maintaineddetuned by a quantity equal to a suitable multiple of the frequencyspacing with respect to the grid of resonances of the other resonantstructures. Preferably, the resonant structures have all the samestructure, i.e. the same type and number of resonators. Preferably, eachof the resonant structures 225, 227, 229 of the optical combiner 220 hasthe same structure of the resonant structure 218 of the optical splitter210. For example, given an FSR of the splitter 210 equal to 400 GHz(m=4), than three (m−1) resonant structures 225, 227 may be used havingFSR=400 GHz and having a detuning of the respective grid of resonancesequal to, respectively, 100, 200 and 300 GHz with respect to the grid ofresonances of the splitter 210.

The optical combiner 220 may have a first optical waveguide 221optically connected to the first input port 222 and a second opticalwaveguide 223 optically connecting the second input port 224 to theoutput port 226. In this case, the one or more resonant structures 225,227 may be optically coupled to the first and second optical waveguide221, 223, as shown in FIG. 2, and each resonant structure is opticallyinterposed between the first and second optical waveguide 221, 223 sothat the respective sub-portion is directed from the first opticalwaveguide 221 to the second optical waveguide 223 by physicallypropagating across the respective resonant structure 225, 227, 229. Incase of a plurality of resonant structures 225, 227, 229, the abovelayout is said ‘in parallel configuration’.

The working point (i.e. the position of the resonances) of the resonantstructures 225, 227, 229 may need to be controlled, either at a fixedposition (‘trimming’), for example in order to properly match theresonances with the grid of the WDM frequencies and/or to compensate forpossible fabrication errors, or dynamically changed (‘tuning’),typically within a tuning range equal to the FSR of the splitting device210, depending on the operative conditions.

In an embodiment, a single tuning device (not shown) is coupled to theoptical combiner 220 in order to control the working point of all theresonant structures 225, 227, 229 at a time. In this case, the resonantstructures may be manufactured in such a way that at a given operativecondition (e.g. a given temperature) the respective resonances of theresonant structures are properly shifted with respect to the resonancesof all the other resonant structures by a quantity equal to thefrequency spacing. This can be achieved in practice by suitably trimmingthe structure of the optical waveguide constituting the microringresonators (e.g. by e-beam dose trimming during lithography or UV-curingof a suitable cladding).

In another embodiment, a tuning device (not shown) is coupled to each ofthe resonant structures 225, 227 in order to control the respectiveposition of the resonances.

As shown in FIG. 2, according to the present invention an optical filter260 is optically coupled to the second optical path 240 and it isadapted to receive at least a portion of the optical radiationpropagating through the second optical path 240 via an input port and tooutput a transformed optical radiation via an output port according tooptical transfer functions (such as phase and power transfer functions).The optical filter 260 may be any optical device having its opticaltransfer functions wavelength-dependent in the wavelength band ofinterest. For example, it may be apt to filter an optical channel withinsaid second portion propagating through the second optical path 240.

The optical filter 260 may be a resonant optical filter, i.e. itcomprises one or more resonant cavities (or resonators), such as Bragggratings or microcavities such as linear cavities, microrings,racetracks, photonic band gap cavities and the like. In a preferredconfiguration, the resonant optical filter 260 comprises microring orracetrack resonators. The transfer functions (e.g. phase, dispersion orpower) of such a resonant optical filter 260 are typically characterizedby strong wavelength dependence at and in the proximity of a resonantwavelength of one or more of its resonators. The perturbations of thepower transfer function (hereinafter called resonances of the opticalfilter) are typically equally spaced in frequency and, in analogy withthe definition given for the splitter 210, the distance between twoadjacent resonances of an optical filter is referred to as the ‘freespectral range’ of the resonant optical filter. In case all theresonators comprised within the optical filter have the same FSR,typically the FSR of the optical filter coincides with the FSR of thesingle resonators.

In a preferred configuration, the optical filter 260 is a tunableoptical filter, i.e. it is apt to select an arbitrary optical channel tobe filtered.

In a preferred embodiment the optical filter 260 is an optical addand/or drop filter (OADF) having at least a further optical port 266(‘drop port’) having the function of dropping or adding, fully orpartially, at least an optical channel within the optical band ofinterest propagating in the optical path 240. In other words, the powertransfer function at the drop port 266 is typically characterized byhigh transmission peaks equally spaced in frequency by a quantity equalto the FSR of the optical filter.

In a preferred embodiment, the OADF 260 has a still further optical port(‘add port’, not shown) which in combination with the further opticalport 266 forms a pair of add and drop ports.

In a preferred configuration, the optical filter 260 has an associatedbypass path 451 suitable to guarantee a hitless tuning of the opticalfilter itself. A first and a second optical switch 452 and 453 mayoptionally be optically coupled to the second optical path 240 and tothe bypass path 451, as shown in FIG. 2. The optical switch 452 isconfigured to assume alternatively a first and a second state, whereinan optical radiation propagating along the optical path 240 is directed,in the first state, substantially solely to the second optical path 240and, in the second state, substantially solely to the bypass path.Preferably, the optical switch 452 switches from the first to the secondstate continuously, i.e. the splitting ratio of the output power of thetwo output ports switches from 0:100 to 100:0 and vice versacontinuously. The second optical switch 453 has the same opticalbehavior of the first optical switch 452, provided that the secondswitch 453 is a “time-reversal copy” of the first optical switch 452,i.e. it operates in reverse with respect to the first switch. In oneadvantageous configuration, the second optical switch 453 is astructurally identical copy of the first optical switch 452 operating inreverse, i.e. the second optical switch 453 is a mirror symmetric copyof the first optical switch with respect to a vertical axis in the planeof FIG. 2. The first and the second optical switch 452 and 453 areconfigured to be operated in synchronous so as to maintain at any timeduring operation a symmetrical reciprocal configuration. The two opticalswitches 452, 453 may be actuated by any actuation technique (such asthermo-optic, MEMS actuated, electro-optic, acusto-optic, elasto-optic,stress, etc) so as to switch from the first to the second state and viceversa.

The first and second optical switch 452, 453 may be any arbitrary devicethat meet the above requirements, including variable couplers (such asplanar waveguide couplers), variable Y branches, Δβ switches,alternating Δβ switches, Mach-Zehnder interferometer (MZI) basedswitches or the like. The first and second optical switch 452, 453 arepreferably wavelength-independent over the allocated WDM bandwidth. Forexample, they may be identical MZI-based optical switches, each onecomprising a balanced MZI having a pair of identical 3-dB opticalcouplers and a controllable phase shifter (for example thermallyactuated) along one of the two arms.

Optionally, an all-pass filter 454 is optically coupled to the bypasspath 451 and it is adapted to introduce a narrowband wavelengthdependent phase change to the optical radiation propagating therethroughso as to match the phase distortion introduced by the tunable opticalfilter 260 at least at a channel neighboring the channel on which theoptical filter 260 is tuned. The all-pass filter 454 comprises aresonant optical cavity. Strong wavelength dependence of the phasetransfer function typically occurs in correspondence of the resonantwavelength(s). The all-pass filter has, in the wavelength band ofinterest, a wavelength independent power transfer function and a phasetransfer function having a wavelength/frequency dependence whichexhibits typical resonance induced behavior. Advantageously, theresonant all-pass filter 454 is apt to be tuned so that at least one ofits resonant wavelengths overlaps to a resonant wavelength of thetunable optical filter 260 on the opposite path 240. Advantageously, theall-pass filter 454 is adapted to have a FSR selectable to be equal tothe FSR of the tunable optical filter 260 so as to facilitatefabrication and phase matching. The resonant all-pass filter 454 isadapted to apply the correct phase distortion on channels adjacent tothe filtered one while leaving substantially unaffected the signalamplitude.

In a preferred configuration, the all-pass filter 454 comprises a singleresonator with a power coupling coefficient between the latter and thebypass path 451 advantageously selected to be equal to the sum of thepower coupling coefficients of the stages (see below) of the filter 260cascaded along the path 240. A single resonator all-pass filter 454 withthe above characteristics helps minimizing the chromatic dispersionintroduced by the all-pass filter.

An advantage of the combination of the optical splitter 210 above andthe optical filter 260 is that the requirements of the optical filter interms of roll-off are relaxed because the optical filters receiveportions of the WDM signal having a coarser grid (e.g. 200 GHz or 300GHz instead of 100 GHz). This allows for example the use of a dropfilter having two-ring stages, as described below, instead of filtershaving three- or four-ring stages, which exhibit much more fabricationand operation challenges.

In the following, a method for optical filtering according to anembodiment of the present invention will be described. This method maybe implemented by operation of the scheme of the optical device 200 ofFIG. 2, described above. Where useful for the understanding of themethod of the present invention, reference will be made to elements andcorresponding reference numerals of FIG. 2, without restricting thescope of the method. The method is particularly suitable to filter atleast an optical channel within a plurality of WDM channels, whileleaving the thru channels with a minimum alteration or no alteration atall.

First, a WDM optical signal comprising a plurality of optical channelshaving respective optical frequencies lying on a grid (‘WDM grid’) ofallocated frequencies equally spaced by a given frequency spacing, saidgrid occupying an optical bandwidth BW, is split by way of the opticalsplitter 210, into a first and a second portion spatially separated. Itis noted that the WDM optical signal does not necessarily need tocomprise all the channels which may occupy said grid until it is filled.Actually, one or more of the allocated frequencies of the grid may bevacant.

Nevertheless, the method and device of the present invention is suitablefor processing a full-grid WDM signal and the examples in the presentdescription will refer to this case, without limiting the scope of theinvention.

The second portion of the optical signal comprises a sub-group of saidoptical channels having optical frequencies lying on a second sub-gridof the WDM grid having frequencies spaced by an integer multiple of saidfrequency spacing and the first portion comprises the remaining opticalchannels lying on a respective first sub-grid of frequencies. The firstand second frequency sub-grids, respectively associated to the first andsecond portion, are complementary sub-grids of the grid of allocated WDMfrequencies described above. Exemplarily, for m−3 the first portion maycomprise the channels selected one on three channels and the secondportion the remaining two channels on three. In each portion, theresidual optical power of the channels substantially belonging to theother portion with respect to the optical power of the first portionchannels (‘cross-talk’) is below −10 dB. Preferably, the cross-talk isbelow −15 dB, more preferably below −20 dB. In the second portion, thecross-talk of the channels substantially belonging to the first portionmay be worse than the cross-talk in the first portion (as shown in FIG.4).

A channel belonging to the second portion is filtered by way of anoptical filter 260, adapted to act solely on the second portion, andconfigured so that one of its resonances overlaps the optical frequencyof said filtered channel. Preferably, one of the two adjacent resonancesoverlaps an optical frequency of the first sub-grid and the otheradjacent resonance lies outside said optical bandwidth. In aconfiguration, both the two adjacent resonances overlaps an opticalfrequency of the first sub-grid.

In case a by-pass path 454, together with switches 452 and 453 ispresent, the filtering of the channel is accomplished by acting on theoptical switches 452 and 453 so as to maintain substantially all theoptical radiation output from the output port 216 of the opticalsplifter 210 on the second optical path 240, so as to interact with theoptical filter 260.

The first portion propagates along the first optical path 230.

The first and second portions of optical channels are then recombined byway of the combiner 220, which is properly tuned or trimmed so as toresonate, as a whole, with the optical frequencies of the first sub-gridand not to resonate with those of the second sub-grid, as previouslydescribed.

With reference now to FIG. 3, a further realization of a tunable opticaldevice 10 in accordance with the present invention will be described.Where appropriate, the same reference numerals of FIG. 2 for likeelements have been used and, for these elements, reference is made tothe description above.

In the configuration shown in FIG. 3, the optical splitter 210 has a FSRequal to the double of the frequency spacing and the optical combiner220 has the same structure of the optical splitter 210. In the presentdescription, the expression ‘having the same structure’ means having thesame type and number of resonators, as well as the same layout of theresonators.

In FIG. 4, there are shown the calculated optical power response curvesfor the first output port 214 (curve 800) and the second output port 216(curve 810) of an exemplary optical splitter 210 as shown in FIG. 3,which comprises two series-coupled microring resonators. Here the zerofrequency conventionally corresponds to a frequency of a channelbelonging to the second portion, i.e. a channel substantially output atthe second output port 216. A rigorous transfer matrix approach and a 3DFinite Difference Time Domain approach have been respectively used forthe calculation of the transfer functions and of the actual dimensionallayout of the optical components of the present description. Throughoutthe present description, the TE polarization mode has been investigated,without restricting the scope of the present invention.

Silicon has been selected as core material of the waveguidesconstituting both the resonators 218 and the optical waveguides 211,213. The choice of silicon is due to its high thermo-optic effect whichenables a high degree of tunability. Silica may be used as a claddingmaterial surrounding the silicon waveguide core, e.g. in a buriedwaveguide. Alternatively other kind of materials could be used ascladding such as: polymers, spin on glass i.e. HSQ, Si3N4, etc. The highindex contrast waveguide obtained by the above material systems allowsfabricating microring resonators with very small radius and negligiblebending losses. Si waveguides height may suitably be in the range of100-300 nm and its thickness in the range of 200-600 nm. In the examplerelevant to FIG. 4, silicon waveguide cross section is about 500 nmwidth and 220 nm height. A Sio2 top cladding with a refractive index ofn_(clad)=1.446 has been included in the design. Silicon refractive indexhas been taken equal to 3.476.

In calculating the optical responses, it has been assumed a realisticvalue for the total loss of the substantially straight (i.e. negligiblebending radiation losses) silicon waveguides (e.g. 211, 213) and of themicroring waveguide 218 of respectively 3 dB/cm and 10 dB/cm. Thepresent invention equally applies in case of different values of losses.The calculated effective and group indexes of the Si waveguide wererespectively in the range of about 2.43-2.48 and 4.21-4.26. The ringradius of the resonators 218 is 55(±1%) μm which corresponds to an FSRof about 200 GHz. The ring to bus and ring to ring power couplingcoefficients are respectively 74% (±5%) and 44% (±5%), which may beexemplarily obtained by a ring to bus gap equal to 120 nm and a ring toring gap equal to 140 nm.

It is noted that the cross-talk at the second output port is better(smaller) than the cross-talk at the first one. The in-band ripple isless than about 0.2 dB and the insertion loss less than about 1 dB.

The optical components described in the present description, such as theoptical splitter/combiner 210/220 and the optical filter 250 or 260 ofFIG. 3, may be fabricated by any fabrication process known in the fieldof integrated optics, such as a layering process on a substrate, e.g. anSOI wafer having a thickness of the buried oxide in the range of 3-10microns and a thickness of the top Si in the range of 50-1000 μm. Thelayering process may include the e-beam lithography and etching steps. ASiO₂ layer could be deposited as a top cladding.

In FIG. 5, it is shown the corresponding dispersion response function ofthe exemplary optical splitter 210 described with reference to FIG. 4.Both the first and the second output port 214, 216 exhibit the samedispersion curve shown in FIG. 5. Here the zero frequency is the same ofFIG. 4 described above. The channels at zero and ±200 GHz are channelsin resonance with the microrings 218 and they have crossed theresonators 218, while those at ±100 GHz are non resonant channels whichremain on the optical path 211/230. It can be seen that the dispersionintroduced by the exemplary optical splitter 210 on the resonantchannels (within the channel bandwidth of ±12.5 GHz around the centralfrequency of each channel) remains below 20 ps/nm, which is anacceptable limit even thought not negligible. On the other hand, thenon-resonant channels are affected by a negligible dispersion. There isconsequently an asymmetry between the channels that are non-resonant andthe ones which are resonant, the latter being disadvantaged in terms oflosses, polarization dependent loss and dispersion.

A maximum value of ±20 ps/nm of the dispersion added to the thruchannels (by the whole optical device 200) is usually specified, while amore relaxed specification (i.e. ±80 ps/nm) is generally required forthe dropped channel(s). This is because the dropped channel is usuallyimmediately detected while the thru channels may travel through severalOADM nodes before being detected so that dispersion accumulation has tobe avoided. With reference to FIG. 3, it is noted that in case theoptical combiner 220 had the same structure of the optical splitter 210and it were tuned, or trimmed, so that its resonances would overlap withthe resonances of the optical splitter 210, than the resonant channels,in operation, would first cross the resonators 218 of the splitter 210and then those 225 of the combiner 220, as described in prior art, e.g.in U.S. Pat. No. 6,839,482. In this case, the non-resonant channelsbelonging to the first portion will propagate along the first opticalpath 230 and will pass thru the optical combiner 220 unaffected and willbe output into the output port 228 corresponding to the first opticalwaveguide 221. The optical channels of the second portion in the secondoptical path 240 would be input to the input port 224 of the combinerand, being resonant with its microrings 225, they would travel acrossthe latter and would be output at the same output port 228. Thus, asclear from FIGS. 4 and 5, they would be affected by twice the dispersionof FIG. 5 due to the microrings. The acquired dispersion of the resonantchannels would sum up twice, thus reaching the maximum acceptable level,while non-resonant channels would be substantially unaffected. The thruoutput would be consequently strongly not equalized and would bedifficult to meet the specification of the filter especially in term ofdispersion.

According to the present invention, the combiner 220 is trimmed by therespective trimming device so that its resonances are detuned infrequency by one half of its FSR (e.g., 100 GHz detuning for a 200 GHzFSR) with respect to the resonances of the optical splitter 210. Inother words, if the optical splitter 210 is configured so as to deviate,in operation, toward the optical path 240 the even channels, the opticalcombiner 220 is configured so as to deviate, in operation, the oddchannels toward the optical path 240 and vice versa. The opticalcombiner 220 is trimmed so that its resonances are interleaved withthose of the optical splitter 210. In other words, if the opticalsplitter 210 resonates at, e.g., the even channels, the optical combiner220 is made to resonate, by the trimming device, at the odd channels.Accordingly, the non-resonant (with respect to the combiner 220)channels propagating along the second optical path 240 will not leavethe optical path 240 and will be output into the output port 226corresponding to the second optical path 240. The resonant (with respectto the combiner 220) channels in the first optical path 230 willpropagate crosswise the resonators of the combiner 220 and will be alsooutput into the same output port 226, as indicated by the thick arrowsin FIGS. 2 and 3. Advantages of this particular configuration is thatevery single channel will propagate crosswise only either the opticalsplitter's resonator(s) or the optical combiner's resonator(s), thushaving the result of an output WDM signal at the output port 226 withmore homogeneous channels than in the prior art device, both in opticalpower (loss, PDL, etc.) and in optical distortion, such as phasedistortion and/or dispersion. The discussion above equally applies tothe optical device 200 of FIG. 2.

In the following, a particular configuration of the optical device 10will be described, which is particularly suitable to add and/or drop achannel. Preferably, the free spectral range of the optical filter 260is substantially equal to an odd multiple of the WDM frequency spacingand greater than half of the WDM optical bandwidth. In other words, theFSR of the optical filter 260 is given by: FSR==(2k+1)Δf±% Δf, being Δfthe frequency spacing and k any positive integer such that k>(BW−2 Δf)/4Δf, being BW the optical bandwidth, or equivalently, k>(N_(ch)−3)/4,being N_(ch) the number of allocated WDM channels. It is noted thatN_(ch)=BW/Δf+1. The term ‘substantially’ used above takes into accountthe ±X % Δf term, wherein X is less than or equal to 50 or, preferably,less than or equal to 40 or more preferably less than or equal to 25.The value of ±50% Δf may be suitable for a 10 Gbit/s NRZ or RZ channelbit-rate having 100 GHz or 50 GHz spacing. However this value may dependon transmission parameters such as the channel bit-rate and thefrequency spacing and it is ultimately determined by the maximumallowable dispersion and/or loss on the channel near the parkedresonance (see below). For a 40 Gbit/s NRZ or RZ channel bit-rate, asmaller value may be suitable, for example equal to +25% Δf. Forexample, for a bandwidth equal to about BW=4000 THz and a frequencyspacing equal to Δf=100 GHz (41 channels), than FSR=(2k+1)100±40 GHz,with k≧10, e.g. FSR=2100±40 GHz.

A further tunable optical filter 250 may advantageously be opticallycoupled to the first optical path 230. The optical filter 250 isadvantageously a resonant optical filter having optical filteringfunctionality similar to those of the optical filter 260. The freespectral range of the further tunable resonant filter 250 issubstantially equal to an odd multiple of the WDM frequency spacing andgreater than half of the WDM optical bandwidth (FSR=(2k+1)Δf±X % Δf,k>(BW-2 Δf)/4 Δf, or equivalently, k>(N_(ch)−3)/4 and X≦50 or X≦40).

Preferably, the FSR of at least one of the first and second opticalfilter 250, 260 exceeds the half of the optical bandwidth by a quantitygreater than the frequency spacing. In other words, k is selected suchthat k>BW/4Δf or equivalently k>(N_(ch)−1)/4. According to theApplicant, the optimal choice for k is (N_(ch)−1)/4+1>k>(N_(ch)−1)/4.Reasons for these selections will be given below. For example, for abandwidth equal to about BW=4000 THz and a frequency spacing equal toΔf=100 GHz (41 channels), than FSR=(2k+1)100±40 GHz, with preferablyk≧11. According to the Applicant, the optimal choice for k is k=11, i.e.FSR=23±40 GHz.

For reasons of easy of manufacture and operation, it could be preferablethat the FSR of the further tunable optical filter 250 has the samecharacteristics of the FSR of the tunable filter 260. Preferably, thefurther optical filter has the same structure of the optical filter 260.Accordingly, in a preferred configuration both the optical filters havethe respective FSR exceeding the half of the optical bandwidth by aquantity greater than the frequency spacing.

The particular filter configuration of the optical device 10 accordingto FIG. 3 allows the FSR of the two optical filter 250, 260 beingadvantageously smaller than said optical bandwidth, i.e. k<(BW−Δf)/2Δf,or equivalently k<(N_(ch)−2)/2. More preferably, k is selected so thatk<(3 BW−4Δf)/8Δf, or equivalently k<(3 N_(ch)−8)/8. In the exampleabove, advantageously, k≦18, and, more advantageously, k≦13.

Optionally, the further optical filter 250 has an associated by-passpath optically coupled to the first optical path 230 by way ofrespective optical switches in the same way as described above withrespect to the optical filter 260. Analogously, an optional all-passfilter may be coupled to the bypass in the same way and with the samefunctions as described above with reference to all-pass filter 454.

The combination of the optical filters 250 and 260 is comprised in afiltering cell 299, e.g. a drop cell 299 having output port 256 and/or266. The device 10 of FIG. 3 is particularly suitable to filter aplurality of independent optical channels arbitrarily chosen in the WDMgrid, by way of suitably cascading a corresponding plurality offiltering cells 299 along the direction of propagation of the opticalradiation in the optical paths 230 and 240. Each filtering cell 299 isapt to filter one channel arbitrarily selected within the whole WDM gridand independently from the channels filtered by the other cells 299. Thecascade of filtering cells 299 is comprised, with respect to saiddirection of propagation, between the optical splitter 210 and theoptical combiner 220. The advantage of this solution is that the thruchannels pass across only either one of the splitter 210 and thecombiner 220 only once, thus seeing limited overall dispersion by thedevice 10.

Exemplarily, the filter cell 299 of the device 10 is a tunable opticaladd and/or drop cell 299 wherein the resonant-type optical filters 250,260 are tunable optical add and/or drop filters (OADF) 250, 260comprising microring resonators. The optical filters 250 and 260 haveexemplarily the same structure and the following description of theoptical filter 260 equally applies to the optical filter 250 (whereinthe first optical path 230 takes the place of the second optical path240). The following description of the optical filter 260 equallyapplies to the optical filter 260 of the optical device 200 of FIG. 2.It is to be understood that the microring resonators of the embodimentof FIG. 3 may be replaced by any suitable resonator, such as racetracks,waveplates, etc. The optical filter 260 comprises a first stage adaptedto drop (or add) a WDM channel from (or into) the optical path 240. Thefirst stage comprises at least a microring 255 optically coupled to theoptical path 240 and to a drop waveguide 267 in series configuration,which means that, in operation, the optical radiation resonant with themicroring propagates from the optical path 240 to the microring and thento the drop waveguide 267. The drop port 266 of the optical filter 260may belong to the drop waveguide 267, as shown in FIG. 3. In theexemplary embodiment of FIG. 3, the first stage of the tunable OADF 260comprises a second microring 258 series-coupled with the first microringbetween the optical path 240 and the drop waveguide 267. Thus, inoperation, the resonant optical radiation first propagates through thefirst microring and then through the second and finally through the dropwaveguide 267. Advantageously, additional series-coupled microrings maybe added within the first stage, e.g., in order to improve the roll-offof the drop function of the optical filter 260.

Optionally, additional microring-based filtering stages may be cascadedalong the optical path 240 in order to improve the optical response ofthe optical filter 260. For example, each of them may be apt to ‘clean’the thru channels (i.e. to further remove the resonant channel from theoptical path 240) and/or to add a further channel, preferably equal tothe dropped one, into the optical path 240, in case the first stage actsas a drop stage. In FIG. 3 a second and a third stage are exemplarilyshown having a configuration and an operating point (i.e. resonancefrequencies) identical to that of the first stage, so as to clean thethru channels.

In FIGS. 6 a and 6 b there are respectively plot the calculated(transfer matrix method) amplitude and dispersion drop response at thedrop port 266 of the three stages—two series-coupled microrings addand/or drop filter 260 described above. As in the previous examples (seeFIGS. 4 and 5) silicon waveguides buried in a SiO₂ cladding werecontemplated, but the OADF 260 can be realized by any suitable opticalmaterial system. Within each stage, the power coupling coefficients ofthe couplers between the bus-waveguides (240 or 267) and the waveguidesconstituting the microrings are equal to about 7.8% (suitable range6.5%-8.5%) and the power coupling coefficient of the coupler between thetwo adjacent microrings is equal to about 0.21% (suitable range0.19%-0.22%). The microring radius is equal to about 4.9(±1%) μm, whichcorresponds to a FSR equal to about 2300 GHz (18.4 nm around 1550 nm).The exemplarily designed second order Chebyshev optical filter 260 meetsthe following specifications: passband (at drop port 266) equal to about35 GHz with drop loss less than about 3 dB, extinction (at drop port266) on adjacent WDM channels (200 GHz spacing) greater than or equal toabout 30 dB. In the exemplary embodiment of FIGS. 6A, 6B, the bus andthe microring waveguides were wide respectively about 400 nm and 490 nmand high about 220 nm, the bus to ring gap was about 130 nm wide and thering to ring gap was about 260 nm wide. All the other design parametershave been assumed equal to the previous examples (FIGS. 4 and 5).

The OADF 260 may be thermally tuned by micro-heater (not shown) placedabove the microrings, e.g. over the SiO₂ upper cladding. Other knowntuning techniques may be used, such as electro-optics, magneto-optics,opto-mechanical and the like.

In one embodiment, a tunable resonant all-pass filter comprising asingle microring resonator 454 coupled to the bypass path 451 is adaptedto be tuned to match the phase distortion introduced by the opticalfilter 260. The all pass filter 454 has the FSR substantially equal tothe FSR of the tunable OADF 260 and the bus-to-ring power couplingcoefficient substantially equal to the coupling coefficient of a singlestage of the OADF 260 times the number of stages of the OADF 260 (threein the example above). The resulting power coupling coefficient for theabove example is equal to about 23.4%.

FIG. 7 shows the power response at the output port 226 of the tunableOADM 10 exemplarily described with reference to FIGS. 3, 4, 5 and 6,wherein the combiner 220 has the resonances shifted by half FSR (i.e.one frequency spacing) with respect to the optical splitter 210,according to the present invention. The filter specifications are met:the extinction ratio at the drop channel is less than 30 dB and theinsertion loss for the thru channels is about 1 dB.

FIG. 8 shows the corresponding dispersion response at the output port226. The dispersion specification of 20 ps/nm is met in correspondenceof the thru channels.

In an embodiment, as shown in FIG. 3, the drop cell 299 may have asingle drop port 298. This can be accomplished when the drop waveguide257 of the OADF 250 and the drop waveguide 267 of the OADF 260 are thesame waveguide. In other words, a common drop waveguide (257, 267) isoptically coupled both to the drop stage of the OADF 250 and to the dropstage of the OADF 260. The drop port 298 may belong to this commonwaveguide. In this configuration, one of the two OADF (e.g. 260, asshown in FIG. 3) is placed downstream the other optical filter (e.g.,250) with respect to the direction of propagation of a dropped opticalradiation along the common drop waveguide. In other words, OADF 260 isplaced downstream the output port 256 of the optical filter 250. In thiscase, care should be taken that the downstream OADF does not insert inthe optical path 240 the channel dropped by the upstream OADF, thusremoving the dropped channel from the common drop waveguide. For thisreason, it is preferable that the OADF having an FSR exceeding the halfof the optical bandwidth by a quantity greater than the frequencyspacing is the downstream OADF. In case the upstream OADF is dropping achannel at or near the center of the bandwidth, this solution allowsparking the parked resonance of the downstream OADF on a channel havingthe same parity of the dropped channel but different from the latter.Moreover, a common drop waveguide allows a single drop port in presenceof the two OADFs 250, 260 without adding loss to the dropped channel,which for example arises when using a conventional 3-dB coupler.

Optionally, a drop cleaning stage 270 may be coupled to the common dropwaveguide (257, 267) to further clean the dropped channel. For example,the role of the drop cleaning stage may be to remove the residualoptical power, if any, in correspondence of the “parked” resonance ofone of the two OADFs 250, 260.

In the following, a method for optical filtering according to anembodiment of the present invention will be described. This method maybe implemented by operation of the scheme of the optical device 10 ofFIG. 3, described above. Where useful for the understanding of themethod of the present invention, reference will be made to elements andcorresponding reference numerals of FIG. 3, without restricting thescope of the method. The method is particularly suitable to filter atleast an optical channel within a plurality of WDM channels, whileleaving the thru channels with a minimum alteration or no alteration atall.

First, a WDM optical signal comprising a plurality of optical channelshaving respective optical frequencies lying on a grid (‘WDM grid’) ofallocated frequencies equally spaced by a given frequency spacing, saidgrid occupying an optical bandwidth BW, is split by way of the opticalsplitter 210, into a first and a second portion spatially separated.

The first portion of the optical signal comprises a sub-group of saidoptical channels having optical frequencies lying on a first sub-gridhaving frequencies spaced by the double of said frequency spacing andthe second portion comprises the remaining optical channels lying on arespective second sub-grid of frequencies. The first and secondfrequency sub-grids, respectively associated to the first and secondportion, are complementary sub-grids of the grid of allocated WDMfrequencies described above. Exemplarily, the first portion may comprisethe channels having even parity and the second portion the channelshaving odd parity. In the following, the expression ‘belonging to thefirst/second portion’ is equivalent to the expression ‘havingfirst/second parity’. In each portion, the residual optical power of thechannels substantially belonging to the other portion with respect tothe optical power of the first portion channels (‘cross-talk’) is below−10 dB. Preferably, the cross-talk is below −15 dB, more preferablybelow −20 dB.

The optical splitter 210 is operated, during the splitting, so that itsresonant structure 218 is tuned so as to resonate with the opticalfrequencies of the second sub-grid, i.e. the channels of the secondportion overlaps the resonances of said resonant structure 218.

An initial channel belonging to the first or second portion is filtered,e.g. by way of an optical filter adapted to act solely on the first orsecond portion and tuned so that one of its resonances overlaps theoptical frequency of said initial channel.

The first and second portions of optical channels are then recombined byway of the combiner 220. The optical combiner 220 is operated, duringthe combining, so that its resonant structure 225 is tuned so as toresonate with the optical frequencies of the first sub-grid, i.e. thechannels of the first portion overlaps the resonances of said resonantstructure 225.

Assuming the initial channel belongs to the first portion, preferablythe optical filter 250 adapted to act solely on the first portion hasone of the two adjacent resonances overlapping an optical frequency ofthe second sub-grid and the other adjacent resonance lies outside saidoptical bandwidth.

This is pictorially illustrated in FIG. 9A, wherein the horizontal axisrepresents the optical frequency. The left thick arrow represents thefirst allocated optical frequency of the WDM grid (Channel 1,conventionally taken as the origin of the axis, i.e. at 0 GHz) and theright thick arrow that of the last allocated frequency (Channel N,exemplarily at 4000 GHz for the 41^(st) channel of a 100 GHZ spacingfull-grid signal). The thin arrows represent the resonances of theoptical filter 250. The arrow 600 represents the ‘active resonance’,i.e. that resonance referred to above which overlaps the opticalfrequency of the initial channel to be filtered belonging to the firstportion. Exemplarily, the initial channel may be the 26^(th) (even)channel at 2500 GHz. An adjacent resonance 610 (called ‘inactiveresonance’) overlaps any allocated optical frequency of the secondsub-grid (exemplarily, assuming an FSR of the optical filter 250 equalto 2300 GHz, the optical frequency at 200 GHz, i.e. the odd thirdchannel). This has the advantage that, owing to the fact that no or verysmall optical power (related to the cross-talk) is present at thisresonant frequency, the optical filter 250 typically interacts weakly,or not at all, with the optical power in correspondence of thisfrequency. If this interaction is not negligible, additional measuresmay be taken, as described below. In addition, being the resonance 610placed in between two adjacent even channels, they are not significantlyaffected in terms of amplitude, phase and dispersion by the opticalfilter 250.

The other adjacent resonance 620 (called ‘out-of-band resonance’) ismade to lie outside the optical bandwidth occupied by the grid ofallocated frequencies (exemplarily at frequency 2500+2300=4800 GHz) andconsequently it does not interact with the optical channels.

Occasionally, depending on the value of the FSR, it may happen that alsothe first adjacent resonance is made to lie outside the opticalbandwidth occupied by the grid of allocated frequencies, for example incase the initial filtered channel lies at or in the proximity (i.e.within the range ±|FSR−BW/2−Δf|) of the center bandwidth. In the exampleabove, where FSR=2300 GHz and center bandwidth BW/2=2000 GHz, in casethe even filtered channel lies in the range 1800-2200 GHz (i.e. 1900 or2100 GHz), both the two adjacent resonances falls outside the WDMbandwidth.

The feature that the FSR of the first and second optical filter 250 and260 of FIG. 3 is an odd multiple of the channel spacing and also greaterthan half of the occupied bandwidth, in combination with the firstoptical splitter splitting odd and even channels respectively to thefirst and second optical filter, allows to filter an arbitrary channelin an optical bandwidth while tuning the first and the second opticalfilter 250 and 260 by an FSR which may be smaller than said opticalbandwidth (in the example above 2300 GHz instead of 4000 GHz for thesecond optical filter 260 and 2100 GHz or 2300 GHz for the first opticalfilter 250).

In case a by-pass path, with respective switches, is present, thefiltering of the initial channel is accomplished by acting on theoptical switches so as to maintain substantially all the opticalradiation output from the output port 214 of the optical splitter 210 onthe first optical path 230, so as to interact with the optical filter250.

With reference to the second portion, at least one of the following twosteps is performed.

1) The second portion is made to bypass a second optical filter 260,which is adapted to act solely on the second portion, and no interactionarises with it. This may be accomplished, with exemplary reference toFIG. 3, by properly actuating the switches 452 and 453 so as to directthe second portion to the bypass path 451. Preferably, the secondoptical filter 260 is tuned with one of its resonance in the proximityof the center of the WDM bandwidth. This solution 1) is preferable incase no all-pass filter 454 is present.

2) In case the second portion, e.g. by properly actuating the switches452 and 453, is maintained onto the second optical path 240, the secondoptical filter 260 is tuned so that one of its resonances (referred toas the ‘parked resonance’) overlaps an optical frequency of the firstsub-grid at or in the proximity of the center of the optical bandwidthof the WDM grid and the two respective adjacent resonances both lieoutside said optical bandwidth (‘out-of-band resonances’). This ispictorially illustrated in FIG. 9B, having the same conventional symbolsof FIG. 9A. The thin arrows represent now the resonances of the opticalfilter 260. The arrow 700 represents the ‘parked resonance’ on top of anoptical frequency of the first sub-group and the arrows 710 and 720represent the two adjacent resonances which are made to lie outside theoptical bandwidth occupied by the allocated frequencies grid andconsequently they do not interact with the optical channels. Owing tothe fact that no or very small optical power (related to the cross-talk)is present at the parked resonance frequency, the optical filter 260typically interacts weakly, or not at all, with the optical power incorrespondence of this frequency. If this interaction is not negligible,additional measures may be taken, as described further below.

In the example above, when also the second optical filter 260 has an FSRof about 2300 GHz, the parked resonance may correspond to the frequencyof the 22^(nd) (even) channel at 2100 GHz or the 20^(th) channel at 1900GHz in order to have both the two adjacent resonances falling outsidethe WDM bandwidth. This is the reason why the FSR of one of the twooptical filters 250 and 260 (in the example above the second opticalfilter 260) is selected so as to exceed the half of the opticalbandwidth by a quantity greater than the frequency spacing (k>BW/4Δf orequivalently k>(N_(ch)−1)/4). In fact, assuming that the total number ofallocated channels on the WDM grid is odd, in case the channel to befiltered is even, then the parked resonance of the inactive filter (i.e.the filter which is not filtering any channel and which is apt to actsolely on the portion comprising the odd channels) needs to be parked onan even channel near the central frequency (which corresponds to an oddchannel) of the bandwidth. Assuming the best case of an even channeladjacent to the central channel, the smallest distance from the two endsof the bandwidth is equal to half of the optical bandwidth plus thefrequency spacing and thus the FSR of this filter preferably exceedsthis quantity (e.g. FSR=BW/2+2Δf). On the other end, in case the channelto be filtered is odd, then the parked resonance of the inactive filter(i.e. the filter apt to act solely on the portion comprising the evenchannels) needs to be parked on an odd channel which may beadvantageously chosen as that corresponding exactly to the centralfrequency of the bandwidth. In this case, it is enough that the FSR ofthis optical filter (in the example above the first optical filter 250)exceeds the half of the optical bandwidth (e.g. FSR=BW/2+Δf).

In case the total number of allocated channels on the WDM grid is even,by suitably selecting the respective parked resonance as close aspossible to the center of the WDM bandwidth, it is sufficient that theFSR of both the optical filters 250 and 260 is greater than half of thebandwidth (in addition to be an odd multiple of the channel spacing).For example, given a bandwidth BW=3900 GHz and a frequency spacing equalto Δf=100 GHz (40 channels), than FSR=(2k+1)100±40 GHz, with k≧10.According to the Applicant, the optimal choice for k is k=10, i.e.FSR=2100±40 GHz.

Regarding the choice of performing step 1) or 2) above, it depends onthe presence or not of the by-pass paths of FIG. 3, with or without theall-pass filters, and on trade-off considerations. In case the by-passarm 451 is present without the all-pass filter 454 (e.g. because it isnot strictly necessary for having hitless switching), then it ispreferable, according to the Applicant, to perform step 1) above becausein this case the channels belonging to the second portion do notinteract with the optical filter 260 and they are not affected byadditional distortion along the by-pass path 551.

If the all-pass filter 454 is present for hitless purpose, then anoptimal solution should be found choosing the lower between thedispersion introduced by the all-pass filter on the thru channels (ofthe second portion) when the optical filter is by-passed and thedistortion (loss and/or dispersion) introduced by the optical filter onthe thru channels adjacent the parked resonance when it is notby-passed.

In case the channel to be filtered need to be changed from the initialchannel to a final channel (i.e. tuning of the optical device 10), thefollowing steps may be preferably performed. Preferably, the initial andfinal channels are switched off. In case the final channel belongs tothe same portion of the initial channel, i.e. the first portion, it issufficient to tune, preferably hitlessly (see below), the first opticalfilter until one among the previously active resonance 600, thepreviously inactive resonance 610 or the other adjacent resonance 620overlaps the final channel, depending on the relative position betweenthe frequency of the final channel and those resonances. For example,the resonance 610 may be used to span over the (first portionfrequencies in the) first half of the bandwidth and the resonance 600 tospan over the (first portion frequencies in the) second half of thebandwidth. The second optical filter 260 may not need to be tuned, beingalready parked (or by-passed) on a proper frequency.

It will now be assumed that the final channel belongs to the secondportion, i.e. the other portion with respect to the initial channel.

The second optical filter 260 is tuned until one of its resonances(‘active resonance’) overlaps the optical frequency of the finalchannel, one of the two adjacent resonances (‘inactive resonance’)overlaps an optical frequency of the first sub-grid and the otheradjacent resonance is an out-of-band resonance. Occasionally, it mayhappen that also the first adjacent resonance is made to lie outside theoptical bandwidth. The choice of the active resonance depends on therelative position between the frequency of the final channel and theresonances, as described above.

Preferably, the step above of tuning the second optical filter isperformed hitlessly, e.g. exploiting the by-pass path 454. Assuming thecase of step 1) above (optical filter 260 by-passed), at the end of thetuning of the optical filter 260 the second portion is redirected to thesecond optical path 240 by way, e.g. of the synchronous switches 452 and453. In the further case of using an all-pass filter 454, this is tuned,before having completely actuated the switches 452 and 453, so as tomatch the phase distortion introduced by the optical filter 260, atleast in correspondence of the WDM channels neighboring the final one.This phase matching is achieved at least for the two channelsimmediately adjacent, and having the same parity of, the processed one.Typically, no phase matching is achieved at the frequency of the finalchannel. Typically, the all-pass filter 454 is tuned until one of itsresonant wavelengths overlaps the frequency of the final channel onwhich it is also being tuned the optical filter 260. Then, the opticalswitches 452 and 453 may be synchronously switched so as to direct theWDM second portion from the by-pass path 451 to the second optical path240. In all the intermediate states during the switching operation, thetwo fractions of the second portion propagating respectively along thetwo optical paths remain in a phase relationship which is suitable toproperly recombine in the optical switch 453 so as to be entirelyoutputted in the proper output port (corresponding to the optical path240) of the optical switch 453 without loss and/or distortion.

Assuming the case of step 2) above (optical filter not by-passed), thesecond portion may be first redirected to the by-pass path (possiblyexploiting the all-pass filter 454 as described above), then the opticalfilter 260 is tuned (e.g., the parked resonance 700 may be tuned so asto become an active resonance on a channel belonging to the secondportion) and then the procedure of redirection described above may beapplied.

With reference to the first portion, at least one of the following twosteps is performed.

A) The first portion is made to bypass the first optical filter 250 sothat no interaction arises. This may be accomplished, e.g., by properlyactuating the respective switches so as to direct the first portion tothe respective bypass path. Preferably, the first optical filter 250 istuned with one of its resonance in the proximity of the center of theWDM bandwidth. This solution A) is preferable in case no respectiveall-pass filter is present.

B) The first optical filter 250 is tuned until one of its resonances(‘parked resonance’) overlaps an optical frequency of the secondsub-grid at or in the proximity of the center of the optical bandwidthof the WDM grid and the two respective adjacent resonances both lieoutside said optical bandwidth and consequently they do not interactwith the optical channels. The same considerations above with regard tothe FSR of the optical filter 260 and the hitless tuning may be appliedto the optical filter 250. Again, owing to the fact that no or verysmall optical power (related to the cross-talk) is present at the parkedresonance frequency, the optical filter 250 typically interacts weakly,or not at all, with the optical power in correspondence of thisfrequency. If this interaction is not negligible, additional measuresmay be taken, as described above with reference to stage 270 of FIG. 3.

Optionally, in case they were switched-off, the initial and finalchannels are now switched-on. Said final channel is now filtered by wayof said second optical filter 260; for example it may be dropped.

During the entire operation, the thru channels remain substantiallyunaffected.

Although the present invention has been disclosed and described by wayof some embodiments, it is apparent to those skilled in the art thatseveral modifications to the described embodiments, as well as otherembodiments of the present invention are possible without departing fromthe spirit or essential features thereof/the scope thereof as defined inthe appended claims.

1-30. (canceled)
 31. An optical device comprising: an optical splitterhaving an input port, a first output port, a second output port and aresonant structure comprising at least a resonator, the optical splitterbeing adapted to receive at said input port an optical signal comprisinga plurality of channels lying on a grid of optical frequencies equallyspaced by a frequency spacing and occupying an optical bandwidth, andwherein said optical splitter is adapted to output at said first andsecond output ports, respectively, a first and a second portion of saidoptical signal, said second portion comprising the channels lying on asecond sub-grid of optical frequencies spaced by an integer multiple ofsaid frequency spacing and the first portion comprising the remainingchannels lying on a first sub-grid, wherein the resonant structure iscapable of resonating with the optical frequencies of the second portionso that, in operation, the second portion is output at the second outputport by interaction with the resonant structure; an optical combinerhaving a first input port, a second input port, and an output port andadapted to receive at said first and second input ports, respectively,the first and the second portion and to output the first and secondportions at said output port; a first optical path optically connectingthe first output port of the optical splitter to the first input port ofthe optical combiner and capable of propagating said first portion; asecond optical path optically connecting the second output port of theoptical splitter to the second input port of the optical combiner andcapable of propagating said second portion; and an optical filteroptically coupled to the second optical path and capable of filtering achannel within said second portion propagating through the secondoptical path, wherein the optical combiner comprises at least oneresonant structure comprising at least a resonator and is capable ofresonating with the optical frequencies of the first portion so that, inoperation, the first portion is output at said output port byinteraction with said at least one resonant structure.
 32. The opticaldevice of claim 31, wherein the optical combiner comprises a pluralityof resonant structures, wherein each resonant structure comprises atleast a resonator and said resonant structure is capable of resonatingwith the optical frequencies of a respective sub-portion of the firstportion so that, in operation, the respective sub-portion is output atsaid output port by interaction with the respective resonant structure,each respective sub-portion being different from other sub-portions. 33.The optical device of claim 32, wherein: the optical splitter furthercomprises a first optical waveguide optically connecting the input portto the first output port and a second optical waveguide opticallyconnected to the second output port, wherein said resonant structure isoptically coupled to the first and second optical waveguides and isoptically interposed between the first and the second optical waveguidesso that, in operation, the second portion propagates through theresonant structure while being directed from the first optical waveguideto the second optical waveguide; and the optical combiner furthercomprises a respective first optical waveguide optically connected tothe first input port and a respective second optical waveguide opticallyconnecting the second input port to the output port, wherein theresonant structures are optically coupled to the respective first andsecond optical waveguides in parallel configuration, and each one of theresonant structures is optically interposed between the respective firstand second optical waveguides so that, in operation, the respectivesub-portion of the first portion propagates through the resonantstructure while being directed from the respective first opticalwaveguide to the respective second optical waveguide.
 34. The opticaldevice of claim 32, wherein the number of said plurality of resonantstructures of the optical combiner is equal to said integer multiplediminished by one unit and said integer multiple is greater than two.35. The optical device of claim 31, wherein said integer multiple isequal to two.
 36. The optical device of claim 31, wherein eachsub-portion of the first portion comprises the channels lying on asub-grid of optical frequencies spaced by said integer multiple of saidfrequency spacing.
 37. The optical device of claim 31, wherein each oneof the resonant structures comprises a respective tuning device.
 38. Theoptical device of claim 31, wherein the resonant structures share acommon tuning device.
 39. The optical device of claim 31, wherein thefree spectral range of said optical filter is substantially equal to afurther integer multiple of said frequency spacing, said further integermultiple not having common dividers with said integer multiple.
 40. Theoptical device of claim 31, wherein said optical filter is an add and/ordrop filter comprising a respective optical port adapted to drop or addan optical channel within the second portion.
 41. The optical device ofclaim 39, wherein said optical filter comprises at least a resonatoroptically coupled to, and interposed between, the second optical pathand a drop waveguide so as to be capable of dropping said opticalchannel within the second portion from the second optical path towardsaid drop waveguide.
 42. The optical device of claim 31, wherein saidoptical bandwidth is greater than or equal to about 2 THz.
 43. Theoptical device of claim 31, wherein said optical filter comprisesmicro-ring or racetrack resonators.
 44. An optical communication systemcomprising a transmitter, a receiver, an optical line opticallyconnecting the transmitter and the receiver and an optical deviceaccording to claim 31, wherein the optical device is coupled to theoptical line.
 45. A method for filtering an optical signal comprising aplurality of channels lying on a grid of optical frequencies equallyspaced by a given frequency spacing and occupying an optical bandwidth,comprising: a) splitting said optical signal by way of an opticalsplitter having an input port, a first output port, a second outputport, and a resonant structure, comprising at least a resonator, whereinsaid optical signal is input at said input port and a first portion ofthe optical signal is output at said first output port, and a secondportion of said optical signal is output at the second output port, saidsecond portion comprising the channels lying on a second sub-grid ofoptical frequencies spaced by an integer multiple of said frequencyspacing and overlapping the resonances of said resonant structure andthe first portion comprising the remaining channels lying on the firstsub-grid; b) filtering a channel belonging to the first or secondportions; and c) after filtering, recombining said first portion andsecond portion by way of an optical combiner having a first input port,a second input port, a respective output port, and at least one resonantstructure comprising at least a resonator, wherein the first portion isinput at said first input port, the second portion is input at saidsecond input port, and both the first and the second portions are outputat said respective output port, the channels of the first portion havingoptical frequencies overlapping the resonances of said at least onerespective resonant structure of the optical combiner.
 46. The method ofclaim 45, wherein: said optical splitter further comprises a firstoptical waveguide optically connecting the input port to the firstoutput port and a second optical waveguide optically connected to thesecond output port, and wherein the resonant structure is opticallycoupled to the first and second optical waveguides and opticallyinterposed between the first and the second optical waveguides so thatthe second portion is output at the second output port after crossingthe resonant structure; and said optical combiner further comprises arespective first optical waveguide optically connected to the firstinput port and a respective second optical waveguide opticallyconnecting the second input port to the respective output port, andwherein the at least one respective resonant structure is opticallycoupled to the respective first and second optical waveguides andoptically interposed between the respective first and second opticalwaveguides so that the first portion is output at said respective outputport after crossing the at least one respective resonant structure. 47.The method of claim 45, wherein the resonant structure of the opticalsplitter comprises a plurality of series-coupled resonators.
 48. Themethod of claim 45, wherein said at least one resonator of the resonantstructure of the optical splitter and/or of the optical combiner is amicro-ring or racetrack resonator.
 49. The method of claim 45, whereinthe optical combiner has the same type and number of resonators of theoptical splitter.
 50. The method of claim 45, wherein the step offiltering is performed on the second portion by way of an optical filteradapted to act solely on the second portion and configured so that oneof its resonances overlaps the optical frequency of said filteredchannel, one of the two adjacent resonances overlaps an opticalfrequency of the first sub-grid, and the other adjacent resonance liesoutside said optical bandwidth.
 51. The method of claim 50, wherein thedistance between said one resonance of said optical filter and each oneof said two adjacent resonances exceeds half of the optical bandwidth.52. The method of claim 51, wherein the distance between said oneresonance of said optical filter and each one of said two adjacentresonances exceeds half of the optical bandwidth by more than saidfrequency spacing.
 53. The method of claim 50, wherein the opticalfilter comprises at least a resonator optically coupled to, andinterposed between, an optical path and a drop waveguide so that thestep of filtering comprises dropping said channel within the secondportion from said optical path toward said drop waveguide.
 54. Themethod of claim 45, wherein said integer multiple is equal to two. 55.The method of claim 54, further comprising at least one of the twofollowing steps: making the first portion bypass a further opticalfilter adapted to act solely on the first portion; and configuring saidfurther optical filter so that one resonance of the optical filteroverlaps an optical frequency of the second sub-grid near the center ofsaid optical bandwidth and the two respective adjacent resonances bothlie outside said optical bandwidth.
 56. The method of claim 55, whereinthe further optical filter comprises at least a respective resonatoroptically coupled to, and interposed between, a further optical path anda respective drop waveguide so as to be capable of dropping an opticalchannel within the first portion from the further optical path towardsaid respective drop waveguide.
 57. The method of claim 56, wherein thedrop waveguide of the further optical filter and the drop waveguide ofthe optical filter are the same waveguide so that the further opticalfilter and the optical filter share the same drop waveguide.
 58. Themethod of claim 55, wherein the further optical filter has the samestructure of the optical filter.
 59. The method of claim 45, whereinsaid optical bandwidth is greater than or equal to about 1 THz.
 60. Themethod of claim 45, wherein step c) of filtering the channel comprisesdropping said channel.